This invention relates to a cryogenic treatment of landfill gas to reduce its content of moisture and a mixture of undesirable vapors, such as halogenated, aromatic and oxygen-containing organic compounds. More particularly, the invention involves compressing landfill gas to a high pressure and then expanding it isenthalpically to a cryogenic temperature to effect the separation of undesirable compounds therefrom.
The gas formed in many landfills is drawn to utilize its methane content which is generally at least about 50% by volume. About 90% of the remainder of the gas is carbon dioxide. The total composition of landfill gas includes moisture and very small or trace amounts of halogenated organic compounds such as dichloroethane, aromatic hydrocarbons such as toluene, and oxygen-containing hydrocarbons such as acetone; this variable miscellany of compounds that are mostly air pollutants will hereinafter be referred to by the collective term, troublesome compounds.
Landfill gas is currently utilized either as fuel in a combustion engine or as feed to a plant that separates it into high-purity methane and even high-purity carbon dioxide. U.S. Pat. Nos. 4,252,548 and 4,704,146 teach the recovery of methane and carbon dioxide, respectively, from landfill gas.
The gas from landfills has, to date, been utilized mostly as fuel in combustion engines in spite of the serious corrosion that necessitates frequent repairs of each engine, often less than four months apart. To the cost of such frequent repairs must be added the financial loss inherent in the down-time.
The rapid corrosion of combustion engines running on landfill gas is associated with the aforesaid miscellany of troublesome compounds. Upon combustion in an engine, every chlorinated hydrocarbon, such as chloro-benzene, yields very corrosive hydrogen chloride.
Accordingly, a principal object of this invention is to provide a cryogenic process of treating landfill gas to materially reduce its corrosiveness when fed to combustion engines.
Another important object is to minimize the escape of air-pollutants such as hydrogen halides into the atmosphere when landfill gas is burned.
A further object is to provide a process of chilling landfill gas to sub-freezing temperatures after adding methanol to the gas.
A still further object is to recover and recycle the methanol used in chilling landfill gas to sub-freezing temperatures.
These and other features and advantages of the invention will be apparent from the description which follows.